1. Field of the Invention
The present invention relates to an apparatus used in the assembly of roller cone rock bits, and in particular to an apparatus providing thermal control of the bit during welding operations so as to prevent damage to the components of the bit.
2. The Prior Art
Roller cone rock bits have been widely used in the petroleum industry for many years. The bits themselves have been subjected to many changes and modifications which have improved the life of the bit. However, the bits have largely remained in the category of a disposable item. That is to say that once the bit or any portion thereof is damaged or worn to the point that the bit can no longer function effectively, the entire bit is typically discarded or scrapped to be replaced with a new bit. Clearly, this is an expensive procedure.
Typically, any wear or damage to the bit occurs near or in the roller cone assemblies. This is not unexpected as the working surfaces of the cones themselves engage the rock face, and the bearings receive great loads. The upper portion of the bit is primarily a large metal member having an outer profile for mating with a tool joint and includes interior bores for the delivery of fluids to lubricate the bearings and flush out the debris cut from the formation by the bit. The upper portion of the bit, by its shear size, represents a considerable portion of the overall cost of a bit. Thus, if the upper portion can be salvaged and reused, there should be considerable savings over the cost of a new bit.
In typical new bit manufacture, three bit body pieces, each extending the full length of the bit body to be formed, and each including one leg and a sector of the tool joint, are welded together along juncture areas extending generally lengthwise of the bit body, so that the central circulation bore is actually formed during the welding process.
U.S. Pat. Nos. 3,850,250 and 4,158,973 suggest methods whereby legs with cones mounted thereon are welded to a full annular upper body portion to form a complete bit. The juncture surfaces thus welded are substantially normal to the central rotational axis of the bit. The patents suggest the use of high energy beam welding techniques, such as electron beam welding, since these techniques are very fast. However, there is a problem of heating in any welding technique. Welding massive metal bodies, such as drill bits, can cause sufficient heating of the metal at the juncture surfaces to transmit enough heat through the bit to damage the bearings and/or the seals surrounding them. This is a particular problem in the methods suggested by U.S. Pat. Nos. 3,850,250 and 4,158,973 because the transverse juncture areas are close to, and sometimes even aligned with, the seals.
One possible solution would be to use heat resistant seals. However, seals can be made heat resistant only at the expense of other properties which are highly desirable in bit seals. When using high energy beam welding techniques, particularly electron beam welding, as suggested by these prior patents, the bit body should be rotated during welding, and the welding is typically performed in a vacuum chamber. These factors would tend to complicate efforts to cool the bit during welding.